1. Field of the Invention
The present invention relates to magnetic resonance imaging (MRI) devices, and more particularly to MRI devices designed to image various human extremities.
2. Description of Related Art
Magnetic resonance imaging (MRI) is a known technique in which an object, animate or inanimate, is placed in a spatially varying magnetic field and subjected to pulses of radiation at a frequency which causes nuclear magnetic resonance in the object, the spectra obtained thereby being combined to form cross-sectional images of the object. The MRI technique is especially useful for medical or veterinary applications because living tissues contain an abundance of hydrogen atoms, whose nuclei resonate at radio frequencies. An MRI apparatus thus operates in general by the application of a radio frequency (RF) field in the presence of other magnetic fields, and the subsequent sensing and analysis of the resulting nuclear magnetic resonances induced in the body.
Any nucleus which possesses a magnetic moment tends to align itself with the direction of the magnetic field in which it is located. Accordingly, when a substance such as human tissue is subjected to a static magnetic field, the individual magnetic moments of the protons in the tissue attempt to align with this polarizing magnetic field. However, the protons precess around the direction of the field at a characteristic angular frequency, know as the Larmor frequency, which is dependent on the strength of the magnetic field and the properties of the specific nuclear species. Once in the polarizing magnetic field, the alignment of the protons exist in one of two possible energy states which describe the spin angular momentum of the protons. Classically, the protons precess, i.e., each proton's axis of rotation generally describes a cone and tends to turn at an angle relative to the direction of the applied polarizing magnetic field. The phases of rotation of the proton are essentially random and a net macroscopic magnetic moment is therefore produced in the direction of the polarizing field, with the randomly oriented magnetic components in the perpendicular or transverse plane cancelling one another.
When the tissue or substance is subjected to an RF radiation pulse which is in the plane transverse to the polarizing magnetic field and which is at or near the Larmor frequency, the net aligned moment is rotated or tipped into the transverse plane to produce a net transverse magnetic moment in the transverse plane at or near the Larmor frequency. The processing protons at this time are no longer random in phase, but rather in a single phase orientation. The degree to which the net magnetic moment is tipped, and hence the magnitude of the net transverse magnetic moment, depends primarily on the duration of time and the magnitude of the applied RF radiation signal.
When the radiation pulse is terminated, the protons realign with the polarizing magnetic field. The resulting changing magnetic moment is measurable, and the magnitude of the radiation emitted by the realigning protons is related to the proton density of the tissue being imaged and its relaxation times (the time necessary for the protons to realign themselves with the polarizing magnetic field), the number of protons in turn being an indicator of the density of the substance (mostly H.sub.2 O) in the case of humans. The radiation generated by the relaxation of the moments induces a current or electro-motive force (EMF) signal according to Faraday's law in an antenna positioned to enable a series of images of the tissue to be obtained and processed.
Various types of receiving antennas or coils have been designed for MRI applications. The most commonly utilized antenna is the standard sized whole body coil which is dimensioned to be disposed around the entire body of the patient to be imaged, the patient being placed in a tubular member or tunnel which supports the coil and confines the patient during the procedure. This arrangement has several disadvantages. First, due to the standard sizing, a significant void or empty region is defined between the coil and the portion of the patient to be imaged. As the imaged portion of the patient becomes a smaller fraction of the coil's volume, the signal-to-noise ratio decreases, thereby degrading the image quality. Second, the coil receives resonance signals from a significantly larger area than the region of interest, resulting in a sensitivity to extraneous information which degrades the spatial resolution and increases aliasing in the two and three-dimensional Fourier transform methodology used in processing. Third, the configuration of the coil is less than optimal in terms of efficiency or the quality and homogeneity of the RF field generated. Finally, many individuals do not feel comfortable when placed in the tube for extended periods of time, especially those individuals who exhibit claustrophobic tendencies.